The present invention relates to a projection exposure apparatus used to transfer a mask pattern onto a photosensitive substrate in a photolithography process for producing, for example, semiconductor devices, image pickup devices (CCDs, etc.), liquid-crystal display devices, or thin-film magnetic heads. More specifically, the present invention relates to a scanning projection exposure apparatus, e.g. a step-and-scan projection exposure apparatus, which has a catadioptric system as a projection optical system and performs exposure by synchronously scanning a mask and a photosensitive substrate relative to the projection optical system.
To produce semiconductor devices, for example, a projection exposure apparatus is used to transfer a pattern formed on a reticle as a mask onto each shot area on a wafer coated with a photoresist. Hitherto, a step-and-repeat type (one-shot exposure type) reduction projection exposure apparatus (stepper) has frequently been used as a projection exposure apparatus for the pattern transfer process. On the other hand, to meet the demand that the area of a pattern to be transferred should be increased without substantially increasing the load on the projection optical system, attention has recently been paid to a step-and-scan type projection exposure apparatus wherein a reticle and a wafer are synchronously scanned relative to a projection optical system in a state where a part of a pattern on the reticle is projected as a reduced (demagnified) image on the wafer, thereby sequentially transferring the demagnified image of the reticle pattern onto each shot area on the wafer. The step-and-scan method has been developed by combining together the advantage of the transfer method of the aligner (slit scan method) that transfers a pattern on the whole surface of a reticle onto the whole surface of a wafer in the magnification ratio of 1:1 by one scanning exposure and the advantage of the transfer method of the stepper.
In general, it is required for resolution of projection exposure apparatuses to be increased. One approach to increase resolution is to use a light beam of a shorter wavelength as an illuminating light for exposure. Accordingly, use has recently been made of excimer laser light in the ultraviolet and far-ultraviolet regions as illuminating light for exposure, such as KrF excimer laser light (wavelength: 248 nm) or ArF excimer laser light (wavelength: 193 nm). The use of metal vapor laser light, higher harmonics of YAG laser light, etc. has also been examined.
When excimer laser light is used as illuminating light for exposure, for example, broadened-band laser light sources and narrowed-band laser light sources are available as excimer laser light sources. The term xe2x80x9cnarrowed-band laser light sourcexe2x80x9d means a laser light source in which the spectral half-width of laser light is not more than 2 pm to 3 pm. The term xe2x80x9cbroadened-band laser light sourcexe2x80x9d means a laser light source in which the spectral half-width of laser light is not less than 100 pm. When illuminating light of a short wavelength in the ultraviolet region or shorter wavelength region, such as excimer laser light, is used, vitreous materials usable for refracting lenses of projection optical systems are limited to such materials as quartz and fluorite. Therefore, as the wavelength of illuminating light used shortens as described above, it becomes more difficult to achromatize the projection optical system. Accordingly, it is desirable to use a narrowed-band laser light source in order to facilitate achromatization of the projection optical system.
However, the band of excimer laser light is originally broad. Therefore, in narrowed-band laser light sources, the oscillation spectral width of excimer laser light is narrowed by injection locking or the like. For this reason, the laser output of narrowed-band laser light sources is lower than that of broadened-band laser light sources. Further, narrowed-band laser light sources are inferior to broadened-band laser light sources in terms of lifetime and production cost. Therefore, in terms of the laser output, lifetime and production cost, broadened-band laser light sources are more advantageous than narrowed-band laser light sources. Accordingly, attempts have recently been made to use a broadened-band laser light source in a projection exposure apparatus having a projection optical system structured such that achromatization can be readily achieved.
Incidentally, projection optical systems usable in scanning exposure type projection exposure apparatuses (scanning projection exposure apparatuses) such as step-and-scan type projection exposure apparatuses include a catadioptric system that uses a concave mirror, and a refracting optical system formed from a combination of refracting lenses only, as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 6-132191. When such a catadioptric system is used, achromatization can be readily achieved by disposing a concave mirror in a group of refracting lenses because concave mirrors are free from chromatic aberrations. Consequently, it becomes possible to use a broadened-band laser light source, which is advantageous in terms of laser output, lifetime, etc.
Even in the case of using the second-mentioned refracting optical system, it is possible to use a broadened-band laser light source because the range of achromatization can be widened by increasing the proportion of fluorite in the entire refracting lens system.
In the above-described prior art, when the second-mentioned refracting optical system is used as a projection optical system, it is necessary to use fluorite for ten-odd lens elements in a total of twenty-odd lens elements, for example, in order to achieve achromatization over a wavelength width of the order of 100 pm to use a broadened-band laser light source. However, fluorite has the following properties: it is difficult to machine; the yield after the machining is unfavorably low; the change of refractive index with temperature is large; and the coefficient of thermal expansion is high, so that deformation occurs to a considerable extent in response to changes in temperature. Tharefore, if many fluorite lenses are used, the temperature dependence of the image-forming characteristics of the projection optical system becomes unfavorably high.
When the first-mentioned catadioptric system is used as a projection optical system, it is possible to achieve achromatization over a wavelength width of the order of 100 pm by disposing a concave mirror in a predetermined position in a group of a predetermined number of refracting lenses, for example, because concave mirrors are free from chromatic aberrations. However, it is necessary in a scanning projection exposure apparatus to set the demagnification ratio for a pattern transferred from a reticle to a wafer on the order of from xc2xc to ⅕, for example, and if a concave mirror is merely disposed in such a group of refracting lenses, the range in which favorable image-forming characteristics can be obtained becomes an arcuate area. Since the pattern areas on reticles have a rectangular external shape, if scanning exposure is carried out with such an arcuate area, the reticle scanning distance must be set to be considerably greater than the width of the pattern area. This causes the reticle-side stage to increase in size, unfavorably.
Further, when such an arcuate area is used, it is also necessary to use a lens that is not axially symmetric, and it is not easy to machine a non-axially symmetric lens with the desired accuracy.
Moreover, when a catadioptric system is used as a projection optical system, the projection optical system becomes large in size and complicated in arrangement because of routing of the image-forming light beam. Therefore, it is desirable to form the whole projection exposure apparatus in as compact a structure as possible by taking into consideration the reticle scanning direction, etc.
Further, it has been pointed out that when ultraviolet light such as excimer laser light is used as illuminating light for exposure, it is necessary to circulate nitrogen (N2) gas or a gas (e.g. air) having ozone removed therefrom in the projection exposure apparatus in consideration for the absorption of ultraviolet light by ozone and also the properties of photoresist. However, if all the gas in a chamber in which the projection exposure apparatus is installed is merely replaced by nitrogen gas or the like, for example, a problem arises in terms of the safety of workmen during maintenance or the like.
Further, it is necessary in a projection exposure apparatus to control the amount of exposure light applied to a wafer according to the sensitivity, etc. of a photoresist used. In this regard, with a method wherein the amount of ultraviolet light emitted as pulsed light, such as excimer laser light, is reduced through an ND filter plate, for example, the ND filter plate may be damaged by intense pulsed light. Further, it is desirable that the amount of ultraviolet light should be capable of being controlled continuously and accurately. However, with the method wherein the amount of ultraviolet light is controlled by using a light-reducing plate such as an ND filter, the amount of light cannot always be continuously set, depending upon the positioning accuracy of the light-reducing plate.
Furthermore, in the conventional step-and-scan type projection exposure apparatus, the projection optical system and the reticle-side stage are secured to a column stood on a surface plate to which a stage (wafer stage) for holding a wafer is secured. Accordingly, when the reticle and wafer are synchronously scanned during exposure, vibrations of the stages affect the projection optical system, which should essentially be stationary. This may degrade the image-forming characteristics.
In view of the above-described circumstances, a first object of the present invention is to provide a scanning projection exposure apparatus capable of using as a projection optical system a catadioptric system formed from a combination of refracting lenses which are all axially symmetric and a reflecting optical member, and also capable of obtaining favorable image-forming characteristics.
A second object of the present invention is to provide a projection exposure apparatus designed so that the whole structure of the apparatus can be made compact even when it uses a projection optical system consisting essentially of the above-described catadioptric system.
A third object of the present invention is to provide a projection exposure apparatus designed so that during exposure, the absorption of illuminating light for exposure is minimized, and during maintenance, the safety of workmen can be ensured.
A fourth object of the present invention is to provide a projection exposure apparatus using ultraviolet light as illuminating light for exposure and capable of accurately controlling the amount of ultraviolet light for exposure at all times.
A fifth object of the present invention is to provide a projection exposure apparatus capable of obtaining favorable image-forming characteristics independently of vibrations caused by the synchronous scanning of a reticle and a wafer.
A projection exposure apparatus according to the present invention includes a light source; a mask having a transfer pattern; a projection optical system that projects an image of a part of the transfer pattern onto a photosensitive substrate by light from the light source; and a device that transfers the transfer pattern on the mask onto the photosensitive substrate by synchronously scanning the mask and the photosensitive substrate relative to the projection optical system. The projection optical system includes a first optical system having a concave mirror and is arranged to reflect and condense a light beam from the mask such that the light beam returns toward the mask; a second optical system that directs the light beam, which is returned toward the mask by the first optical system, toward the photosensitive substrate; and a third optical system that forms an image of a part of the transfer pattern of the mask on the photosensitive substrate by the light beam from the second optical system.
The first and third optical systems may be disposed along the scanning direction of the mask and the photosensitive substrate, and the center of gravity of the projection optical system may lie outside the optical path of the image-forming light beam passing through the first and third optical systems.
The projection exposure apparatus may further include an off-axis alignment sensor for detecting the position of a mark for alignment on the photosensitive substrate. The alignment sensor may have an optical system arranged such that an optical axis of the optical system is substantially parallel to the optical axis of the third optical system and a predetermined distance away from the optical axis of the third optical system in the scanning direction of the photosensitive substrate.
The arrangement may be such that the optical axes of the first and third optical systems are parallel to each other, and a surface position detecting system is disposed to lie in a direction perpendicularly intersecting a plane containing the optical axes of the first and third optical systems. The surface position detecting system includes an irradiation optical system that applies a light beam obliquely to the surface of the photosensitive substrate, and a light-receiving optical system that photoelectrically converts the light beam reflected from the photosensitive substrate to obtain a photoelectrically converted signal. The surface position detecting system detects a displacement of the surface of the photosensitive substrate on the basis of the photoelectrically converted signal from the light-receiving optical system.
The arrangement may be such that the second optical system has a partially transmitting mirror that directs the light beam from the first optical system toward the photosensitive substrate and that transmits a part of a light beam from the photosensitive substrate, and that a photoelectric detector is provided to receive that part of the light beam from the photosensitive substrate which passes through the partially transmitting mirror.
The photoelectric detector may be a light-receiving element that photoelectrically converts reflected light from the photosensitive substrate.
The photoelectric detector may be an image pickup device that picks up an image of a fiducial mark in the vicinity of the photosensitive substrate.
The projection optical system may be an image-forming optical system that forms an intermediate image of a part of the transfer pattern of the mask between the mask and the photosensitive substrate. A device for correcting image-forming characteristics of the projection optical system may be provided in the vicinity of a position where the intermediate image is formed.
A projection exposure apparatus according to another aspect of the present invention includes an illumination optical system that illuminates a mask having a transfer pattern formed thereon by illuminating light from a light source; a projection optical system that projects an image of the pattern on the mask onto a photosensitive substrate under the illuminating light; a substrate stage that moves the photosensitive substrate; a plurality of casings that accommodate the illumination optical system, the projection optical system, and the substrate stage independently of each other; and a gas supply device that selectively supplies a plurality of different kinds of gas into at least one of the casings.
The plurality of different kinds of gas may be gases selected from the group consisting of nitrogen, air, and air having ozone removed therefrom.
The gas supply device may be provided with a device for confirming that a gas in a casing to be supplied with another gas has been substantially completely replaced by the substituting gas.
A projection exposure apparatus according to still another aspect of the present invention includes an illumination optical system that illuminates a mask having a transfer pattern formed thereon by illuminating light from a light source; a projection optical system that projects an image of the pattern on the mask onto a photosensitive substrate under the illuminating light; a casing having light-transmitting windows disposed in the optical path of the illuminating light between the light source and the photosensitive substrate; and a gas controller that supplies into the casing a gas in which the amount of ozone per unit volume is variable; wherein the illuminance of illuminating light emitted from the light source and applied to the photosensitive substrate is controlled by changing the amount of ozone in the gas supplied into the casing from the gas controller.
A projection exposure apparatus according to a further aspect of the present invention includes a light source; a mask having a transfer pattern; a projection optical system that projects an image of a part of the transfer pattern onto a photosensitive substrate by light from the light source; a device having a mask-side stage that scans the mask relative to the projection optical system, and a substrate-side stage that scan the photosensitive substrate relative to the projection optical system in synchronism with the mask-side stage, so that the transfer pattern on the mask is transferred onto the photosensitive substrate by synchronously scanning the mask and the photosensitive substrate relative to the projection optical system; an antivibration plate to which a movable part that moves synchronously with scanning exposure is secured; and another antivibration plate to which a stationary part that is not synchronous with scanning exposure is secured.
The projection exposure apparatus may be provided with an interferometer to measure the position of the mask-side stage or the substrate-side stage. The interferometer has a moving mirror mounted on the mask-side stage or the substrate-side stage; a reference mirror mounted on the antivibration plate to which the stationary part is secured; and an interferometer body part that detects a relative displacement between the moving mirror and the reference mirror by using a light beam.
A projection exposure method according to the present invention includes the step of preparing a projection optical system including at least three optical systems, and the step of projecting an image of the pattern on the above-described mask onto the above-described substrate after forming an intermediate image of the pattern in the projection optical system.
In addition, the present invention provides a method of controlling an environment of an exposure apparatus in which an image of a pattern on a mask is projected onto a substrate. The method includes the step of casing at least a part of constituent portions of the exposure apparatus, and the step of supplying a plurality of different kinds of gas into the cased constituent portion of the exposure apparatus.